1640s (in Anglo-Latin from early 14c.), shortening of posse comitatus "the force of the county" (1620s, in Anglo-Latin from late 13c.), from Medieval Latin posse "body of men, power," from Latin posse "have power, be able" (see potent) + comitatus "of the county," genitive of Late Latin word for "court palace" (see comitatus). Modern slang meaning "small gang" is probably from Western movies.